Move to the `git_error` name in the internal API for error-related
functions.

Now that the index has a "dirty" state, where it has changes that have
not yet been committed or rolled back, our tests need to be adapted to
actually commit or rollback the changes instead of assuming that the
index can be operated on in its indeterminate state.

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

When diffing against an index, return a new `GIT_DELTA_CONFLICTED`
delta type for items that are conflicted.  For a single file path,
only one delta will be produced (despite the fact that there are
multiple entries in the index).

Index iterators now have the (optional) ability to return conflicts
in the index.  Prior to this change, they would be omitted, and callers
(like diff) would omit conflicted index entries entirely.

This function has one output but can match multiple files, which can be
unexpected for the user, which would usually path the exact path of the
file he wants the status of.